SOUND
SOUND
H
ow do you come to know that a
‘period’ is over in your school?
You come to know easily that
someone is at your door when he knocks
or you hear the sound of the doorbell.
Most of the time you can make out that
someone is approaching you by just
hearing the foot steps.
You might have played a game called
hide and seek. In this game a person is
blind-folded and has to catch the
remaining players. How is the blind-
folded person able to guess which player
is closest to her?
Sound plays an important role in our
lives. It helps us to communicate with
one another. We hear a variety of sounds
in our surroundings.
Make a list of sounds you hear in
your surroundings.
In the music room of your school you
hear the sounds produced by musical
instruments like flute, tabla,
harmonium, etc. (Fig. 10.1).
How is sound produced? How does
it
travel from one place to another? How
do we hear sound? Why are some sounds
louder than others? We shall discuss
such questions in this chapter.
Fig. 10.1 : Some musical instruments
Tabla
Harmonium
Sitar
Flute
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10.1 Sound is Produced by a
Vibrating Body
Touch the school bell when not in use.
What do you feel? Again touch it when
producing sound. Can you feel it
vibrating?
Activity 10.1
Take a metal plate (or a pan). Hang
it at a convenient place in such a
way that it does not touch any wall.
Now strike it with a stick (Fig.10.2).
Do you hear a sound? Touch the
plate or pan gently with your finger.
Do you feel the vibrations?
Fig. 10.2 : Striking a pan
Again strike the plate with the stick
and hold it tightly with your hands
immediately after striking. Do you
still hear the sound? Touch the
plate after it stops producing sound.
Can you feel the vibrations now?
As you learnt in Class VII the to and
fro or back and forth motion of an
object is called vibration. When a
tightly stretched band is plucked, it
vibrates and produces sound. When it
stops vibrating, it does not produce
any sound.
Activity 10.3
Take a metal dish. Pour water in it.
Strike it at its edge with a spoon
(Fig. 13.4). Do you hear a sound?
Again strike the dish and then
touch it. Can you feel the dish
vibrating? Strike the dish again.
Look at the surface of water. Do you
see any waves there? Now hold the
dish. What change do you observe
on the surface of water? Can you
explain the change? Is there a hint
to connect sound with the
vibrations of a body?
Activity 10.2
Take a rubber band. Put it around
the longer side of a pencil box
(Fig. 10.3). Insert two pencils
between the box and the stretched
rubber. Now, pluck the rubber band
somewhere in the middle. Do you
hear any sound? Does the band
vibrate?
Fig. 10.3 : Plucking the rubber band
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We see that a vibrating object
produces sound. In some cases, the
vibrations are easily visible to us. But
in most cases, their amplitude is so small
that we cannot see them. However, we
can feel them.
Activity 10.4
Take a hollow coconut shell and
make a musical instrument ektara.
You can also make it with the help
of an earthen pot (Fig. 10.5). Play
this instrument and identify its
vibrating part.
Table 10.1 : Musical Instruments and
their Vibrating Parts
S.No. Musical Vibrating Part
Instrument Producing Sound
1. Veena Stretched string
2. Tabla Stretched
membrane
3.
4.
5.
6.
7.
Many of you might have seen the
manjira (cymbals), the ghatam, and the
noot (mudpots) and the kartal. These
musical instruments are commonly
used in many parts of our country. These
instruments are simply beaten or struck
(Fig. 10.6). Can you name a few other
musical instruments of this type?
You too can make a musical
instrument.
Fig. 10.4 :
Vibrating dish produces waves
in water
Fig. 10.6 :
A few more musical instruments
Ghatam
Make a list of familiar musical
instruments and identify their vibrating
parts. A few examples are given in
Table 10.1. Complete rest of the Table.
Fig. 10.5
: Ektara
Manjira
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10.2 Sound Produced by
Humans
Speak loudly for a while or sing a song,
or buzz like a bee. Put your hand on
your throat as shown in Fig. 10.8. Do
you feel any vibrations?
In humans, the sound is produced
by the voice box or the larynx. Put your
fingers on the throat and find a hard
bump that seems to move when you
swallow. This part of the body is known
as the voice box. It is at the upper end
of the windpipe. Two vocal cords, are
stretched across the voice box or larynx
in such a way that it leaves a narrow
slit between them for the passage of air
(Fig.10.8).
Fig. 10.7 : Jaltrang
When we pluck the string of an
instrument, like the sitar, the sound
that we hear is not only that of the
string. The whole instrument is forced
to vibrate, and it is the sound of the
vibration of the instrument that we
hear. Similarly, when we strike the
membrane of a mridangam, the sound
that we hear is not only that of the
membrane but of the whole body of
the instrument.
When we speak, does
any part of our body
vibrate?
Fig.10.8 : Voice box in humans
When the lungs force air through the
slit, the vocal cords vibrate, producing
sound. Muscles attached to the vocal
cords can make the cords tight or loose.
When the vocal cords are tight and thin,
the type or quality of voice is different
Activity 10.5
Take 6-8 bowls or tumblers. Fill
them with water up to different
levels, increasing gradually from
one end to the other. Now take
a pencil and strike the bowls gently.
Strike all of them in succession. You
will hear pleasant sounds. This is
your jaltrang (Fig.10.7).
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from that when they are loose and
thick. Let us see how the vocal cords
function.
Activity 10.6
Take two rubber strips of the same
size. Place these two pieces one
above the other and stretch them
tight. Now blow air through the gap
between them [Fig. 10.9(a)]. As the
air blows through the stretched
rubber strips, a sound is produced.
You can also take a piece of paper
with a narrow slit and hold it
between your fingers as shown in
Fig. 10.9 (b). Now blow through the
slit and listen to the sound. Our
vocal cords produce sound in a
similar manner.
(a)
(b)
Fig. 10.9 (a), (b) : Working of vocal cords
The vocal cords in men are about
20 mm long. In women these are about
15mm long. Children have very short
vocal cords. This is the reason why
the voices of men, women and
children are different.
10.3 Sound Needs a Medium
for Propagation
When you call up your friend who is
standing at a distance, your friend is
able to hear your voice. How does the
sound propagate or travel to her?
Activity 10.7
Take a metal or glass tumbler. Make
sure that it is dry. Place a cell phone
in it. (Remember that the cell phone
must not be kept in water.) Ask your
friend to give a ring on this cell
phone from another cell phone.
Listen to the ring carefully.
Now, surround the rim of the
tumbler with your hands
(Fig. 10.10). Put your mouth on the
Fig. 10.10 : Sound needs a medium to travel
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Place your ear gently on the water
surface (Fig. 10.11). (Be careful : the
water should not enter in your ear.)
Can you hear the sound of the bell?
Does it indicate that sound can
travel through liquids?
opening between your hands.
Indicate to your friend to give a ring
again. Listen to the ring while
sucking air from the tumbler.
Does the sound become fainter
as you suck air?
Remove the tumbler from your
mouth. Does the sound become
loud again?
Can you think of an explanation? Is
it possible that the decreasing amount
of air in the tumbler had something to
do with decreasing loudness of the ring?
Indeed, if you had been able to suck
all the air in the tumbler, you will not
listen any sound. Actually, sound needs
a medium to travel. When air has been
removed completely from a vessel, it is said
that there is a vacuum in the vessel. The
sound cannot travel through a vacuum.
Does sound travel in liquids? Let us
find out.
Activity 10.8
Take a bucket or a bathtub. Fill it
with clean water. Take a small bell
in one hand. Shake this bell inside
the water to produce sound. Make
sure that the bell does not touch
the body of the bucket or the tub.
Oh ! That is how whales
and dolphins might be
communicating under
water.
Fig. 10.11 : Sound travelling through water
Fig. 10.12 : Sound travelling through a
metre scale
Can you hear the sound of the
scratching? Ask your friends
around you if they were able to hear
the same sound?
Let us find out if sound can travel
through solids also.
Activity 10.9
Take a metre scale or a long metal
rod and hold its one end to your
ear. Ask your friend to gently
scratch or tap at the other end of
the scale (Fig. 10.12).
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129
You can also perform the above
activity by placing your ear at one end
of a long wooden or metallic table and
asking your friend to gently scratch the
other end of the table (Fig. 10.13).
Fig. 10.13 : Sound can travel through solids
We find that sound can travel
through wood or metal. In fact, sound
can travel through any solid. You can
perform interesting activities to show
that sound can also travel through
strings. Have you ever made a toy
telephone (Fig. 10.14). Can you say that
sound can travel through strings?
Fig. 10.14 : A toy telephone
We have learnt so far that vibrating
objects produce sound and it is carried
in all directions in a medium. The
medium could be a gas, a liquid or a
solid. How do we hear it?
Fig. 10.15 : Understanding action of an eardrum
10.4 We Hear Sound through
Our Ears
The shape of the outer part of the ear is
like a funnel. When sound enters it, it
travels down a canal at the end of which
there is a thin stretched membrane. It
is called the eardrum. It performs an
important function. To understand what
the eardrum does, let us build a tin-can
model of the eardrum.
Activity 10.10
Take a plastic or tin-can. Cut its
ends. Stretch a piece of rubber
balloon across one end of the can
and fasten it with a rubber band.
Put four or five grains of dry cereal
on the stretched rubber. Now ask
your friend to speak “Hurrey,
Hurrey” from the open end
(Fig.10.15). Observe what happens
to the grain. Why do the grains
jump up and down?
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The eardrum is like a stretched
rubber sheet. Sound vibrations make
the eardrum vibrate (Fig. 10.16). The
eardrum sends vibrations to the inner
ear. From there, the signal goes to the
brain. That is how we hear.
You have already learnt in earlier classes
about the oscillatory motion and its
time period.
The number of oscillations per
second is called the frequency of
oscillation. Frequency is expressed in
hertz. Its symbol is Hz. A frequency of
1 Hz is one oscillation per second. If an
object oscillates 20 times in one second,
what would be its frequency?
You can recognise many familiar
sounds without seeing the objects
producing them. How is it possible?
These sounds must be different to
enable you to recognise them. Have you
ever thought what factors make them
different? Amplitude and frequency are
two important properties of any sound.
Can we differentiate sounds on the basis
of their amplitudes and frequencies?
Loudness and Pitch
Activity 10.11
Take a metallic tumbler and a
tablespoon. Strike the tablespoon
gently at the brim of the tumbler.
Fig. 10.16 : Human ear
Fig. 10.17 : Thermocol ball touching the
vibrating glass tumbler
We must NEVER put a sharp,
pointed or hard thing into our
ear. It can damage the
eardrum. The damaged
eardrum can impair hearing.
10.5 Amplitude, Time Period
and Frequency of a
Vibration
We have learnt that the to and fro motion
of an object is known as vibration. This
motion is also called oscillatory motion.
Eardrum
Inner ear
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Hear the sound produced. Now bang
the spoon on the tumbler and hear
the sound produced again. Is the
sound louder when the tumbler is
struck hard?
Now suspend a small thermocol
ball touching the rim of the tumbler
(Fig. 10.17). Vibrate the tumbler by
striking it. See how far the ball is
displaced. The displacement of the
ball is a measure of the amplitude
of vibration of the tumbler.
Now, strike the tumbler gently
and then a little harder. Compare
the amplitudes of vibrations of the
tumbler in the two cases. In which
case is the amplitude larger?
Loudness of sound is proportional to
the square of the amplitude of the
vibration producing the sound. For
example, if the amplitude becomes
twice, the loudness increases by a
factor of 4. The loudness is expressed
in a unit called decibel (dB). The
following table gives some idea of the
loudness of sound coming from
various sources.
Normal breathing 10 dB
Soft whisper (at 5m) 30 dB
Normal conversation 60 dB
Busy traffic 70 dB
Average factory 80 dB
Above 80 dB the noise becomes
physically painful.
I wonder why my voice
is different from that of
my teacher.
Compare the sound of a baby with
that of an adult. Is there any difference?
Even if two sounds are equally loud,
they differ in some way. Let us see how.
Fig. 10.18 : Frequency determines the
pitch of a sound
The loudness of sound depends on
its amplitude. When the amplitude of
vibration is large, the sound produced
is loud. When the amplitude is small,
the sound produced is feeble.
The frequency determines the
shrillness or pitch of a sound. If the
frequency of vibration is higher we say
that the sound is shrill and has a higher
pitch. If the frequency of vibration is
lower, we say that the sound has a lower
pitch. For example, a drum vibrates with
a low frequency. Therefore, it produces
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a low-pitched sound. On the other
hand, a whistle has a high frequency
and therefore, produces a sound of
higher pitch (Fig. 10.18). A bird makes
a high-pitched sound whereas a lion
makes a low-pitched roar. However, the
roar of a lion is very loud while the
sound of the bird is quite feeble.
Every day you hear the voices of
children and adults. Do you find any
difference in their voices? Can you say
that the frequency of the voice of a child
is higher than that of an adult?
Usually the voice of a woman has
a higher frequency than that of a
man.
10.6 Audible and Inaudible
Sounds
We know that we need a vibrating body
for the production of sound. Can we
hear the sound of all vibrating bodies?
The fact is that sounds of frequencies
less than about 20 vibrations per second
(20 Hz) cannot be detected by the human
ear. Such sounds are called inaudible.
On the higher side, sounds of frequencies
higher than about 20,000 vibrations per
second (20 kHz) are also not audible to
the human ear. Thus, for human ear
,
the range of audible frequencies is
roughly from 20 to 20,000 Hz.
Some animals can hear sounds of
frequencies higher than 20,000 Hz.
Dogs have this ability. The police use
high frequency whistles which dogs can
hear but humans cannot.
The ultrasound equipment, familiar
to us for investigating and tracking
many medical problems, works at
frequencies higher than 20,000 Hz.
10.7 Noise and Music
We hear different types of sounds
around us. Is the sound always
pleasing? Does a sound sometimes
cause discomfort to you? Some sounds
are pleasant to the ear, whereas some
are not.
Suppose construction work is going
on in your neighbourhood. Are the
sounds coming from the construction
site pleasing? Do you enjoy the sounds
produced by horns of buses and trucks?
Such unpleasant sounds are called
noise. In a classroom, if all the students
speak together, what would the sound
produced be called?
On the other hand you enjoy sounds
from musical instruments. Musical
sound is one which is pleasing to the
ear. Sound produced by a harmonium
is a musical sound. The string of a sitar
also gives out a musical sound. But, if
a musical sound becomes too loud,
would it remain melodious?
10.8 Noise Pollution
You already know about air pollution.
Presence of unwanted gases and
particles in air is called air pollution.
Similarly, presence of excessive or
unwanted sounds in the environment
is called noise pollution. Can you list
some sources of noise pollution? Major
causes of noise pollution are sounds of
vehicles, explosions including bursting
of crackers, machines, loudspeakers etc.
What sources in the home may lead to
noise? Television and transistor radio
at high volumes, some kitchen
appliances, desert coolers, air
conditioners, all contribute to noise
pollution.
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Hearing Impairment
Total hearing impairment, which is rare, is usually from birth itself. Partial
disability is generally the result of a disease, injury or age. Children with
impaired hearing need special care. By learning sign language, such children
can communicate effectively. Because speech develops as the direct result of
hearing, a child with a hearing loss may have defective speech also.
Technological devices for the hearing-impaired have made it possible for such
persons to improve their quality of life. Society can do much to improve the
living environment for the hearing-impaired and help them live normal lives.
What are the Harms of Noise
Pollution?
Do you know that presence of excessive
noise in the surroundings may cause
many health related problems. Lack of
sleep, hypertension (high blood-
pressure), anxiety and many more health
disorders may be caused by noise
pollution. A person who is exposed to a
loud sound continuously may get
temporary or even permanent
impairment of hearing.
Measures to Limit Noise Pollution
To control noise, we must control the
sources of noise. How can this be
achieved? For this, silencing devices
must be installed in air craft engines,
transport vehicles, industrial
machines and home appliances.
How can the noise pollution be
controlled in a residential area?
All noisy operations must be
conducted away from any residential
area. Noise producing industries
should be set up away from such
areas. Use of automobile horns
should be minimised. TV and music
systems should be run at low
volumes. T
rees must be planted along
the roads and around buildings to
cut down on the sounds reaching the
residents, thus reducing the harmful
effects of noise pollution.
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KEYWORDS KEYWORDS
KEYWORDS KEYWORDS
KEYWORDS
AMPLITUDE
EARDRUM
FREQUENCY
hertz (Hz)
LARYNX
LOUDNESS
NOISE
OSCILLATION
PITCH
TIME PERIOD
VIBRATION
VOICE BOX
WIND PIPE
WHAT YOU HAVE LEARNT WHAT YOU HAVE LEARNT
WHAT YOU HAVE LEARNT WHAT YOU HAVE LEARNT
WHAT YOU HAVE LEARNT
Ü Sound is produced by vibrating objects.
Ü In human beings, the vibration of the vocal
cords produces sound.
Ü Sound travels through a medium (gas, liquid
or solid). It cannot travel in vacuum.
Ü The eardrum senses the vibrations of sound,
It sends the signals to the brain. This process
is called hearing.
Ü The number of oscillations or vibrations per
second is called the frequency of oscillation.
Ü The frequency is expressed in hertz (Hz)
Ü Larger the amplitude of vibration, the louder
is the sound.
Ü Higher the frequency of vibration, the higher
is the pitch, and shriller is the sound.
Ü Unpleasant sounds are called noise.
Ü Excessive or unwanted sounds lead to noise
pollution. Noise pollution may pose health
problems for human beings.
Ü Attempts should be made to minimise noise
pollution.
Ü Plantation on the roadside and elsewhere can
reduce noise pollution.
Exercises
1. Choose the correct answer.
Sound can travel through
(a) gases only (b) solids only
(c) liquids only (d) solids, liquids and gases.
2. Voice of which of the following is likely to have minimum frequency?
(a) Baby girl (b) Baby boy
(c) A man (d) A woman
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3. In the following statements, tick ‘T’ against those which are true, and ‘F’
against those which are false.
(a) Sound cannot travel in vacuum. (T/F)
(b) The number of oscillations per second of a vibrating object is called
its time period. (T/F)
(c) If the amplitude of vibration is large, sound is feeble. (T/F)
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
(e) The lower the frequency of vibration, the higher is the pitch. (T/F)
(f) Unwanted or unpleasant sound is termed as music. (T/F)
(g) Noise pollution may cause partial hearing impairment. (T/F)
4. Fill in the blanks with suitable words.
(a) Time taken by an object to complete one oscillation is called
__________.
(b) Loudness is determined by the __________ of vibration.
(c) The unit of frequency is __________
(d) Unwanted sound is called __________ .
(e) Shrillness of a sound is determined by the __________ of vibration.
5. A pendulum oscillates 40 times in 4 seconds. Find its time period and
frequency.
6. The sound from a mosquito is produced when it vibrates its wings at an
average rate of 500 vibrations per second. What is the time period of the
vibration?
7. Identify the part which vibrates to produce sound in the following
instruments.
(a) Dholak (b) Sitar (c) Flute
8. What is the difference between noise and music? Can music become noise
sometimes?
9. List sources of noise pollution in your surroundings.
10. Explain in what way noise pollution is harmful to human.
11. Your parents are going to buy a house. They have been offered one on the
roadside and another three lanes away from the roadside. Which house
would you suggest your parents should buy? Explain your answer.
12. Sketch larynx and explain its function in your own words.
13. Lightning and thunder take place in the sky at the same time and at the
same distance from us. Lightning is seen earlier and thunder is heard
later. Can you explain why?
E X E R C I S E S
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Extended Learning — Activities and Projects
1. Visit the music room of your school. You may also visit musicians
in your locality. Make a list of musical instruments. Note down the
parts of these instruments that vibrate to produce sound.
2. If you play a musical instrument, bring it to the class and
demonstrate how you play it.
3. Prepare a list of famous Indian musicians and the instruments
they play.
4. Take a long thread. Place your hands over your ears and get some
one to place this thread round your head and hands. Ask her to
make the thread taut and hold its ends in one hand. Now ask her
to draw her finger and thumb tightly along the thread (Fig. 10.19).
Can you hear a rolling sound like that of a thunder? Now repeat
the activity while another friend stands near both of you. Can he
hear any sound?
Fig. 10.19
5. Make two toy telephones. Use them as shown in Fig. 10.20. Make
sure that the two strings are taut and touch each other. Let one of
you speak. Can the remaining three persons hear? See how many
more friends you can engage in this way. Explain your observations.
Fig. 10.20
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6. Identify the sources of noise pollution in your locality. Discuss with
your parents, friends and neighbours. Suggest how to control noise
pollution. Prepare a brief report and present it in the class.
You can read more on the related topics on the following websites:
l www.physicsclassroom.com/Class/sound/soundtoc.html
l health.howstuffworks.com/hearing.htm
Golconda fort
Did You Know?
Golconda fort, near Hyderabad, is one of the most magnificient
forts in India. It is famous for many engineering and architectural
marvels. One of the marvels is the water supply system. But,
perhaps, more astonishing is a dome near the entrance to the fort.
A hand-clap at a particular point under the dome reverberates
and can be heard at the highest point of the fort, about a kilometre
away. This was devised as a warning system. If a guard saw a
suspicious movement outside the fort, he clapped at the particular
point under the dome, and the army inside the fort was alerted to
the danger of the approaching enemy.
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